The recently characterized sexual dimorphism and developmental changes in circulating levels of Mullerian inhibiting substance (MIS) in the human after birth remain poorly understood. Further clarification of MIS secretion will not only advance our knowledge of sexual differentiation and maturation, but may reveal if MIS is valuable in the assessment of patient with disordered gonadal development and/or pubertal maturation. In males, the elevated MIS levels of childhood fall at puberty in association with rising gonadotropins and increases in testicular size and steroid production (1,2). We hypothesize that the pubertal changes in MIS are an early event that precedes the increases in gonadal steriod production. Given the pulsatility of gonadotropins and the diurnal easily-obained early marker of sexual maturation. In addition, MIS is discriminatory for normal and dysgenetic testes during early childhood (3,4). Consequently, MIS determination may be valuable diagnostically in distinguishing normal delay in pubertal maturation from GnRH deficiency, and primary from secondarily affect gonadal or hypothalamic-pituitary function. Studies examining the hormonal regulation of MIS have yielded conflicting results (5). Because MIS is a Sertolicell product, FSH is the natural candidate to be its predominant modulator. Yet, evidence from the rat suggests that FSH, LH, and testosterone (T) may all pay a role in the regulation of MIS expression, by different mechanisms (6,7). Similar confusion arises in clinical studies in the human; LH,FSH, and T effects are typically difficult to separate in normal children progressing through puberty. We seek to clarify the regulation of MIS by experimentally dissociating the effects of rising LH, FSH, and T on MIS expression in prepubertal males in this experimental paradigm. We will determine if MIS is an early marker of pituitary-gonadal axis activation by documenting longitudinal changes in MIS expression during spontaneous and pharmacologically induced sexual maturation. These studies will evaluate the utility of MIS measurement in the assessment of patients with disorders of pubertal onset and gonadal development and will help us achieve our long-term goals of understanding the regulation and role of MIS postnatally in the reproductive system.